These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Wizards and Demons are able to summon more of themselves, and Necromancers can summon various undead.
Speaking of undead, many are able to drain your stats, either temporarily, or permanently (the latter requiring a trip to the temple to recover). Just try running away when your Strength is almost all gone and it takes 30 seconds per step!
Goddamned Bats - The Smirking Thief. Steals money and/or items then teleports away. Can take two fireballs to the face without flinching. Though luckily enough, if you are fast enough, you CAN find the thief again later in the level, slaughter it brutally, then get your stolen swag back.
Not to mention actual bats, and replicating mobs.
And manticores, especially in the first game when you haven't worked up that much power yet.
Carrion Creepers, too, in the first game. Things will hit you 20 times for every single action you take.
Good Bad Bugs - Casting from hit points lowers the constitution stat (and thus your maximum HP) until you suddenly die, but if done with the beginning spell Magic Arrow, your HP will stop just shy of zero, allowing you to safely decrease your constitution until it suddenly wraps around and becomes maxed out. Thus, when creating a character, one can set constitution as low as possible in order to raise the other stats, then use this to have all near-optimal stats.
Squick: Some of the battle descriptions can be rather graphic.